There are various kinds of foodstuffs containing enzymes these days, in which sake and beer are typical examples. Sake is generally produced through the following steps: in the first step, fermented rice is compressed and filtered to obtain shinshu (green sake); in the second step, this obtained green sake is sterilized by heating and then stored; in the third step, a plural lots of stored sakes are properly mixed to determine the sake quality and the alcohol content is adjusted to the standards; in the fourth (final) step, the thus adjusted sake is again sterilized by heating and then bottled or packed. As described above, sake undergoes the heat-treatment twice in the second and forth steps in the manufacturing process to inactivate and kill bacteria therein, whereby the sake quality is prevented from deteriorating during circulation.
A problem here is that the fresh aroma of green sake is sharply reduced by the heat-treatments. Therefore, a non-heat-treated sake, or fresh sake, preserving the fresh taste and aroma is in great demand. To meet the demand, for example, the fresh sake is circulated in the market by keeping it at low temperature. Such non-heat-treated sake, however, contains enzymes such as .alpha.-amylase and protease, which deteriorate the sake quality. The increased circulation cost due to the low temperature circulation is another problem.
With respect to muddled fruit drinks such as orange juice, it is considered that the cloud of the drink decides the quality of the drink. To maintain this cloud stably, it is required to inactivate pectin esterase. Since pectin esterase is stable to heat, a heat-treatment for inactivating the enzyme must be conducted at high-temperature (88.degree.-99.degree. C. or 120.degree. C.). The heat-treatment at such high temperature, however, deteriorates the relish of the drink.
Some of the inventors of the present invention found a novel method whereby the problem accompanying the conventional method of inactivating enzymes by heat-treatment can be solved, which is disclosed in the Japanese Patent Application No. H6-180844. In the invention entitled "METHOD FOR INACTIVATING ENZYMES, MICROORGANISMS AND SPORES IN A LIQUID FOODSTUFF" disclosed in the application, a liquid foodstuff containing enzymes is contacted with carbon dioxide in a supercritical state to inactivate the enzymes. In detail, a processing tank with the liquid foodstuff contained therein is sealed and the temperature and pressure therein are maintained under predetermined conditions. Then the supercritical fluid formed into micro-particles by a filter is supplied to the processing tank, whereby the enzymes are inactivated. This method not only improves the inactivating efficiency, but also is highly safe since it is only carbon dioxide that contacts the liquid foodstuff. By this method, simultaneously, microorganisms such as lactic acid bacteria can be killed.
In the process or system disclosed in the above application, however, the inactivating or sterilizing process is not conducted continuously, but it is conducted by a so-called batch system, wherein a liquid foodstuff is stored in the processing tank, the inactivating processing is conducted on the liquid foodstuff in the tank, and then the tank is opened to air to take out the processed liquid foodstuff. Besides its low processing efficiency, the batch processing system is accompanied by a problem that, when the processing tank is opened to air, air including oxygen comes in the processing tank and the product quality deteriorates in the following process of production and after the production. To solve the problem, an additional step of removing air or oxygen must be employed in the process.
A system for continuously processing liquid foodstuffs is disclosed in the Japanese Published Unexamined Patent Application No. H4-222576, in which an extraction from a liquid foodstuff is conducted using a supercritical fluid. In the system, smelling components are extracted from soy sauce by contacting the soy sauce with a supercritical fluid. In more detail, the liquid foodstuff and the supercritical fluid of carbon dioxide are contacted in a counter-flow produced in a processing tank to remove the smelling components from the soy sauce.
It may be considered to apply the counter-flow contacting method to a continuous processing system for inactivating enzymes or sterilizing. A test, however, proved that the inactivating efficiency is very low in the counter-flow contacting method. Thus, a continuous processing system capable of inactivating enzymes with high efficiency and suitable for mass-production has been long desired.
Accordingly, the first object of the present invention is to provide a continuous processing method and system using supercritical fluid of carbon dioxide and having higher inactivating efficiency or sterilizing efficiency than the conventional continuous processing system using counter-flow contacting method.
When a supercritical fluid contacts a liquid foodstuff, not only the enzymes and bacteria in the liquid foodstuff are inactivated and killed, but also some flavor components are often extracted as a gas from the liquid foodstuff and are released to the outside of the processing tank together with the supercritical fluid through a supercritical fluid releasing system. This occurs also in a batch processing system for inactivating enzymes, such as disclosed in the above-cited patent application. As to undesirable components, as in the case of the smelling component of soy sauce which some people dislike, there is no problem. When, on the other hand, favorable components in the liquid foodstuff such as the fresh flavor of sake or juice is concerned, it is not preferable to let even a small amount of such components escape from the liquid foodstuff. Hence, the second object of the present invention is to provide a method and system whereby a flavor component extracted from a liquid foodstuff together with a supercritical fluid during a continuous process of inactivating enzymes or killing bacteria with the supercritical fluid can be retrieved. Thus the liquid foodstuff can be produced with its preferable components preserved even after the inactivating processing, or the flavor components can be taken out from the liquid foodstuff separately.
The third object of the present invention is to provide a method and system whereby the supercritical fluid of carbon dioxide used for inactivating enzymes and killing bacteria can be recovered and the running cost of the continuous processing system is minimized.